1. Field of Invention
The present invention relates to electrically operated heating devices, and is more particularly concerned with such devices having a heating element mounted in a fabric, such as heating pads, blankets, clothing, and the like.
2. Prior Art
Heating pads disclosed in the prior art have been of several general forms. In one form, commonly known as the spiral construction form, a strand of resistance wire, commonly covered with asbestos insulation, has one end at the center of the pad fastened in place by sewing, and arranged in a spiral pattern, the spiral continuing until the pad reaches the desired size. In another form, the strand of resistance wire is in a sinuous pattern, running back and forth completely across the pad, or lengthwise of the pad, until sufficient elongated runs have been placed on the pad to give it the desired length. In both of these forms of construction, the standard practice is to sew the resistance wire in place between two piles of canvas or the like. This pattern interferes with uniform heating over the entire area of the heating pad and commonly causes what is termed "hot spots" in the pad. Pads made according to either of these forms are relatively satisfactory for some purpose, if used to supply low or moderate heat while lying flat, such as for the purpose of warming beds. However, where it is desired to supply heat to a surface or area that is not substantially level, these pads have not proved to be wholly satisfactory, largely due to their stiffness and rigidity with resulting inability to follow the contours of the surface to be heated. The difficulty involved, due to such stiffness and rigidity when the pad is in use, is normally referred to as "bridging."
Heat is commonly prescribed as a means of relieving congestion, pain, soreness, or injury. In order to apply heat effectively, the heat supplying mechanism must readily conform to the contour of the surface to be treated. This calls for a heating mechanism having a pliability and normally high temperature capability that has not been available or even possible in some of the prior art electric heating pads or blankets.
In U.S. Pat. No. 2,154,184, a heating pad is disclosed and claimed having a structure in which the heating element is in the form of small backward and forward sinous loops, several rows of loops being utilized to provide a heating element for the entire pad. Such structure is considerably more flexible than those previously disclosed and provided a pad which was sufficiently flexible to conform to the contours of the various parts of the body. Additionally, thermostats are incorporated in the pad and wired in series with the heating wire element. The thermostats are placed intermediate the rows of heating element loops. Although the heating element disclosed in the above-referred-to patent, constitutes a material improvement over the prior art devices, it still has several drawbacks in that there is a large temperature lag between the thermostat and the heating elements so that before the pad warms up sufficiently to provide suitable control by the thermostat, the temperature of the heating pad first rises to an undesirably high temperature. Additionally the asbestos insulation utilized for insulating the resistance wire often becomes unraveled or torn, resulting in the danger of short circuits and electric shocks.